Role of Central and Peripheral Circulatory Adjustments in Oxygen Transport at the Onset of Exercise

The t 1/2 V̇O2 on has been shown to range from 13 to 30 s for isolated muscle, running dogs, and man. Under the same conditions, the circulatory response preceded the adjustment of V̇O2 , and was judged not to limit V̇O2 . More recently t 1/2 V̇O2 on has been shown to be significantly slower (t 1/2 = 70 to 120 s) for untrained individuals especially in the supine posture. The purpose of the present study was to examine the role played by central, Q̇, and peripheral, MBF, circulatory adjustment in setting t 1/2 V̇O2 on. Q̇, HR, Pa, V̇O2 , and La for eight subjects were measured during rest and exercise at 1.0 for arms and legs and 1.8 and 2.3 1 · min for arm and leg pedaling in the supine posture. Three subjects exercised in the erect and supine posture at the same two work loads. In another study three fit and three unfit subjects exercised at the same work loads with arms and legs in the supine posture while MBF, V̇O2 , and HR were determined continuously at rest and exercise. The t 1/2 V̇O2 on ranged from 24 s to 84 s depending on exercise and fitness. The t 1/2 Q̇ for the same conditions ranged from 6 to 15 s and in virtually every case was less than t 1/2 V̇O2 on. The changes in HR and P̄a followed Q̇. The adjustment of MBF was complete in 6 to 18 s for fit and 30 to 40 s for unfit subjects, e.g., before V̇O2 reached t 1/2. Based on these observations, Q̇ and MBF do not play a role in setting V̇O2 kinetics. The limitation therefore must be distal to the capillary circulation.